Project Summary Communication between cells within multicellular organisms is critical to fundamental aspects of the development of those organisms, the normal functioning of those organisms, and many instances of cancer and disease when those communication processes go awry. Receptor tyrosine kinases (RTKs) are a class of cell surface receptor molecules that ?hear? external signals and transduce those signals into specific cellular responses. In many cases, RTKs can mediate a number of different cellular responses to their stimulation by external signals. The ability of an RTK to mediate a specific response to an external signal, among these different possible cellular responses, is referred to as ?signaling specificity.? Fibroblast growth factors, acting through a subfamily of RTKs known as fibroblast growth factors receptors (FGFRs), are responsible for initiating many different types of biological events, including cell proliferation, angiogenesis, differentiation, cell migration, and cell survival. The long-term objective of this proposal is to understand the molecular basis of signaling specificity by FGFRs. The study of the EGL-15 FGFR in the roundworm Caenorhabditis elegans, a model genetic organism, has long served as a paradigm for understanding the process of FGFR signaling. The C. elegans FGFR, EGL- 15, mediates several distinct processes, including the chemoattraction of the two myoblast precursors of the egg-laying muscles (sex myoblasts, or SMs), and the regulation of the balance of fluid within the organism (fluid homeostasis). Defects in these processes result in striking phenotypes that provide powerful genetic tools that have been used to discover many components that mediate RTK signaling. Some, however, are still missing. This proposal seeks to identify and characterize the ?missing? molecular links between the C. elegans FGF receptor, EGL-15, and the signaling cassettes that allow the specific signal transduction complexes to help mediate specific signaling outcomes. Two domains in EGL-15 are known to mediate specific signaling outcomes: an extracellular domain that is the product of an alternatively-spliced exon5 (the 5A or 5B domains); and the carboxy-terminal domain (CTD) of the intracellular portion of EGL-15. In Specific Aim 1, we will use molecular and genetic approaches to identify sequences within the 5A and 5B domains that are essential for their specific functions, and then use these to help identify components that interact specifically with the 5A or 5B domains. In Specific Aim 2, we will characterize an existing set of 34 new mutations, 28 of which were isolated as enhancers of a mutant that specifically lacks the CTD, that define at least two new components that interact with EGL-15. Whole-genome sequencing will expedite the identification of these potential new components of the EGL-15 signaling complex. This multidisciplinary effort, which uses molecular biology, genome-editing, biochemistry, genetic, and bioinformatics approaches, will enhance our understanding of FGFR signaling specificity.